1. Field of the Invention
The present invention relates to a method of entering a digitized audio signal into a data processing apparatus, such as a personal computer, method of transmitting the audio signal, and audio signal transmitting apparatus and audio signal receiving and reproducing apparatus which permit transmission of the audio signal according to this signal transmitting method.
2. Prior Art
When a computer receives stereo audio signals, analog signals may be input via analog input terminals, and converted into PCM (Pulse Code Modulation) data in the inside of the computer, or digital audio signals may be input via serial ports, such as RS-232C, or SCSI (Small Computer System Interface) ports, for example. In the former method, however, the input signals in the analog format are likely to be affected by noise from the computer. The latter method has a problem of a slow input speed, and, in most cases, cannot be used for exclusively inputting audio signals.
In view of the above situation, an audio input interface has been developed which is designed to convert digitized audio signals into serial data, and transmit the serial data via an exclusive digital audio input terminal, such as SPDIF, as shown in FIG. 1.
In the conventional audio interface as shown in FIG. 1, analog audio signals SL, SR of left (L) and right (R) channels are converted into digital signals by respective A/D converters 101, 102, and packeted by a packet circuit 103 to be converted into serial data, which is then transmitted to a personal computer 200. On the side of the personal computer 200, the received serial data are synchronized by a PLL (phase-locked loop) circuit 201, and the resultant packet data is decoded by a decoding circuit 202 and reassembled into original digital data. Thus, the circuit arrangement tends to be complicated, with a result of an increased cost.
The other method of transmitting digitized audio signals may be roughly classified into a method of transmitting the input signals after converting them into analog signals, and a method of transmitting the digital signals as they are. FIG. 2 shows an example of the former method. In this example, digital audio signals of L channel and R channel which are reproduced in a reproducing system (not shown) are converted into corresponding analog audio signals by respective D/A converters 301L and 301R, and then amplified by respective amplifiers 302L and 302R. The analog audio signals of the two channels obtained from the amplifiers 302L and 302R are then transmitted over respective cables 303L and 303R, amplified by amplifiers 304L and 304R, and then generated from speakers 305L and 305R.
FIG. 3 shows one example of the method of transmitting audio signals in the form of digital signals. In the example of FIG. 3, a packet of digital audio signals of L channel and R channel is transmitted from a digital output circuit 401. This packet is transmitted to a demodulator circuit 403 over a cable 402, and is demodulated by this demodulator circuit 403 into the original digital audio signals of the L channel and R channel. These digital audio signals of the respective channels are converted into analog audio signals by D/A converters 404L and 404R, amplified by amplifiers 405L, 405R, and then generated from speakers 406L and 406R.
The above-described prior art method of FIG. 2, in which the audio signals are transmitted as analog signals, suffers from deterioration of the quality of sound reproduced from the speakers if noise is superposed on the analog signals during their transmission. Where this method is employed within digital equipment, such as a personal computer, in particular, noise involved in the digital signal system is likely to be superposed on analog audio signals during their transmission, whereby the quality of reproduced sound is considerably deteriorated. On the other hand, the method of FIG. 3 is advantageous in that the transmitted audio signal, which is a digital signal, is highly resistant to noise. To implement this method, however, there is a need to provide the digital output circuit for assembling and transmitting a packet of digital audio signals, and the demodulator circuit for demodulating this packet, which may increase the size and cost of the whole apparatus.